(1) Field of the Invention
The present invention related to a novel alignment film material for a liquid crystal cell and a method for preparing the same and, more particularly, to N-aralkyl substituted poly-m-phenylene isophthalamide (PMIA) derivatives of formula 1 which produce a low pretilt angle of less than 1.degree. when the derivatives are applied as a material for an alignment film of the liquid crystal cell, and the method for preparing the same. ##STR1## (wherein, R is an aralkyl group.)
(2) Description of the Related Art
Generally, a liquid crystal cell comprises two rigid substrates and liquid crystal materials injected therebetween. Also, a transparent electrode is formed on the substrate so as to apply an electric field to a pixel of the liquid crystal cell, and an alignment film is formed on the transparent film so as to align the liquid crystal material between the substrates.
In order to obtain the electro-optical effect of the liquid crystal cell, uniform alignment of the liquid crystal materials in the liquid crystal cell is needed. There are two types of the liquid crystal alignment, one is a homogeneous alignment and the other is a homeotropic alignment. The homeotropic alignment aligns the liquid crystal material vertically relative to the substrate surface by depositing SiOx or by coating a surfactant such as a silane based coupling agent on the substrate. The homogeneous alignment aligns the liquid crystal material substantially horizontally relative to the substrate, and performed by a conventional slanted evaporating method of silicone dioxide (SiO.sub.2), a rubbing method, and a Langmuir-blodgett method.
The rubbing method is the most widely used method for aligning the liquid crystals, and comprises the steps of coating polyimide to form an organic alignment film on the substrate and rubbing the alignment film mechanically. By the rubbing process, a lot of micro-grooves are produced on the surface of the alignment film in the rubbed direction. Thus, when liquid crystal is placed on the rubbed alignment film, the liquid crystal is aligned due to the micro-grooves.
Generally, polyimide has been used predominantly as the liquid crystal alignment film material for the rubbing method because of its good liquid crystal aligning property and chemical stability. Furthermore, polyimide is easily printed on the substrate and rubbed. The process for preparing the polyimide alignment film on the substrate comprises the steps of reacting a diamine compound with an acid anhydride in a solvent so as to produce a polyamic acid solution; coating 4-8% solution of the polyamic acid on a substrate; and baking the substrate in a furnace at 180.degree. C.-300.degree. C. The thermoplastic polyimide film obtained by above-process is represented by the formula 2. ##STR2##
However, there is a disadvantage in that the method requires annealing of the coated precursor, polyamic acid, to form a thin polyimide film. In addition, the polyimide can not produce a pretilt angle of less than 1.degree. which is required in an IPS (In-Plane Switching) mode liquid crystal cell recently introduced for a wide viewing angle.